The present invention relates to an underspeed actuator unit which is responsive to operating speed of a prime mover or engine in order to automatically adjust a variable drive coupling in a transmission driven by the prime mover. In particular, the transmission is contemplated as being of a hydrostactic type including one or more pump units with the underspeed actuator being operable to adjust displacement in the pump or pumps.
Hydrostatic transmissions adapted for track-type vehicles commonly include parallel pump units each having variable displacement in order to permit operator control over vehicle speed and direction by appropriately varying displacement of one or both pumps. The present invention is particularly described below in connection with a hydrostatic transmission of this type. However, it will be apparent from the following description that the present invention is also applicable to more conventional hydrostatic transmissions having a single pump unit or multiple pump units whose displacement is always simultaneously varied.
In most hydrostatic transmissions, it is desirable to reduce the load placed upon the prime mover by the transmission when operating speed of the prime mover falls below a selected range because of an external load, such as when a vehicle including the prime mover and transmission is operating uphill or otherwise encounters an increased load.
The use of underspeed regulator or actuator valves is known in connection with hydrostatic transmissions for overcoming this problem may be seen by reference to U.S. Pat. No. 3,477,225 and U.S. Pat. No. 3,727,628, both of which are assigned to the assignee of the present invention. Such underspeed units have been reasonably successful in achieving their objectives while tending to be relatively complex, thus increasing initial and operating costs for the hydrostatic units. In particular, such prior art underspeed valves have commonly been connected to servo valves for regulating variable displacement of the hydrostatic pump units through hydraulic actuator devices. This particular combination has been found to be relatively costly and complex at least in comparison to the present invention.